Crystal plating monitoring system

ABSTRACT

This disclosure relates to crystals, and particularly to plating techniques for improving the quality of crystals. More particularly, this disclosure relates to the production of crystals having a minimum of unwanted, or inharmonic, modes of oscillation. This disclosure describes an apparatus and a method for taking a mode spectrograph to determine the impedance characteristic of a crystal in the frequency range of a desired harmonic while the crystal is in the process of being plated in a vacuum.

United States Patent 1 1 Lukaszek 51 May 15, 1973 1 CRYSTAL PLATINGMONITORING SYSTEM [21] App]. No.2 216,032

[52] U.S. Cl. ..118/8, 118/9, 118/49.l [51] Int. Cl ..C23c 13/08 [58]Field of Search ..118/7, 8, 9, 5, 48-495;

117/1 NQ; 204/1 NQ; 250/1 NQ; 324/56; 310/93, 8.2, 8.9; 73/67.1

3,017,525 l/1962 Wolfskill ..310/9.4 3,059,611 10/1962 Fury et al...118/8 3,382,842 5/1968 Steckelmacher et al. ..118/8 3,383,238 5/1968Unzicker et a1 117/106 R 3,593,125 7/1971 Wilheim et al. ..324/563,600,675 8/1971 Primary ExaminerM0rris Kaplan Attorney-Harry M.Saragovitz [57] ABSTRACT This disclosure relates to crystals, andparticularly to plating techniques for improving the quality ofcrystals. More particularly, this disclosure relates to the productionof crystals having a minimum of unwanted, or inharmonic, modes ofoscillation. This disclosure describes an apparatus and a method fortak- [56] R f en Cit d ing a mode spectrograph to determine theimpedance characteristic of a crystal in the frequency range of a UNITEDSTATES PATENTS desired harmonic while the crystal is in the process of1,649,828 11/1927 1111116 ..324/56 bemg Plated a vacuum- 2,470,7375/1949 Bach ..324/56 X 2,621,624 12 1952 Chil0wsky.... ..118/8 3 Glam, 3Draw; Fgures 2,906,235 9/1959 Hirsh ..118/8 FREQUENCY COUNTER J 25 SWEEPFREQUENCY ATTENUATOR Lgiflililrrl-ljlglc GENERATOR Q ELECTRONIC 0 sweepGENERATOR OSCILLOSCOPE Grenier ..324/56 X PATENTEUHAYI 1s 3,732,546

17 FIG. 1 3|A 46 FREQUENCY SUPPLY CouNTER 34 VACUUM 25 SWEEP YBR oOGARITHMIC FREQUENCY H 1 L SIGNAL ATTENUATOR BRIDGE AMpL| HER GENERATOR1 Q ELECTRONIC SWEEP GENERATOR osc IL LOSCOPE 26 27 FIG 2 FIG. 3

CRYSTAL PLATING MONITORING SYSTEM BACKGROUND OF THE INVENTION Crystalsare very well known and are very valuable, particularly in filters forfrequency control. The techniques for grinding and preparing crystals toobtain the most accurate frequency, the highest possible Q," and theminimum of unwanted modes of oscillation are also well known. However,regardless of the care taken in processing crystals with present,state-of-the-art techniques, there is almost inevitably some degree ofimperfection and even the finest crystals will have a certain amount ofthese unwanted modes of oscillation.

The shear mode of oscillation is through the thickness of the crystaland provides the strong fundamental and harmonic characteristics thatare normally used in the upper high frequency, and very high frequencyranges. However, in addition to the harmonic modes of oscillation, thereare always some unwanted, inharmonic modes of oscillation which, forexample, may be propagated laterally through the crystal. Theseinharmonic or parasitic modes have a somewhat predictable, but notprecisely predictable, relationship to the frequency for which thecrystal is ground or to each other. These inharmonic modes may sometimesbe strong enough to make the crystal, apparently, respond to frequenciesother than those of its fundamental or its harmonic modes, and therebyproduce errors or false settings of frequency in the equipment wherethese crystals are used. They are quite undesirable and the lower theratio between these inharmonic modes and the desired harmonic modes ofthe crystal, the better its operation.

To obtain a strong, clean, main-mode response in quartz resonators, itis necessary to lower the resonant frequency of a portion of the waferbelow the resonant frequency of the surrounding portions in the mannertaught by the Energy-trapping theory. This has the effect of containingthe vibratory energy in a limited region of the quartz wafer, thusenhancing the Q of the resonator. This also has the effect of containingthe group of inharmonic modes of oscillation, which are functions of thelateral dimensions of the electrodes and plate and the mechanicalcharacteristics of the frequency lowered electrode region. The techniquefor lowering the resonant frequency of a portion of the wafer is toplate the parallel surfaces of that portion of the crystal, after it hasbeen ground, to a precise thickness dependent on electrode diameter andresonator frequency. The plating has the effect of introducing separatecut-off frequencies for the electrode and surrounding regions of thewafer, thereby controlling the wave propagation patterns.

However, even though it is established that the general characteristicsof the crystal will be improved by plating, the effect of the plating isnot linear for all the inharmonic modes, and may be different for theinharmonic modes around each of the harmonics. It is, currently, notpossible to predict how the magnitude of each individual inharmonic modewill be effected by the given amount of plating. For criticalapplications, it is still necessary to take all available crystals, asnow fabricated, and test each one, individually, over a band offrequencies around the harmonic of interest, to find the crystal thathas the minimum or an acceptably low level of unwanted modes ofoscillation.

This testing is obviously tedious, time-consuming, expensive, and, atbest, it can only indicate the apparently-best crystal among thosetested. Furthermore, there is no assurance that this is the best qualitythat these crystals, particularly the plated crystals, could achieve.There is always the question whether any of the crystals could have beenimproved by additional plating or could have had the same or betterresults with less plating. This procedure also raises the question ofwhat to do with the less-desirable crystals.

It is therefore an object of this invention to provide an improvedsystem for reducing unwanted modes within a given frequency range of acrystal.

It is a further object of this invention to provide an improved, dynamicmeans for indicating the characteristics of all of the modes ofoscillation of a given crystal while decreasing the unwanted modes ofthe crystal by plating the crystal to obtain the optimum performance forthe crystal.

SUMMARY OF THE INVENTION be carried on simultaneously, this shows theinstantaneous effect of the plating on the modes of oscillation of thecrystal and makes it possible to stop the plating process at a precisepoint whereat the unwanted modes of oscillation of the crystal have allbeen reduced below a given level or whereat the ratio of the desiredfundamental or harmonic mode to the unwanted modes is within anacceptable proportion. The desired harmonic of the crystal can also bemonitored so that the plating process can be stopped before there is asignificant change in the desired harmonic.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawing shows apictorial representation of the crystal plating device with a blockdiagram of the circuitry.

FIGS. 2 and 3 show typical examples of the frequency characteristics ofa crystal, during the course of the plating of the crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG.1, the vacuum, crystal-plating apparatus 10 has a base 11 that supportsa bell jar 12, and a component mount 13 that supports a crystal 14. Thecrystal is electrically coupled through a rotary joint 15 to a coaxialtubing 16 that extends through a vacuum-tight connection 17 to a hybridbridge 24 that is part of a sweep-frequency test circuit 20. Thesweepfrequency test circuit includes a sweep-frequency signal generator21 that connects through an attenuator 23 to the hybrid bridge 24. Alogarithmic amplifier 25 connects the hybrid bridge to an oscilloscope27 with a display tube 28. The sweep of the oscilloscope is synchronizedwith that of the sweep-frequency signal generator by an electronic sweepgenerator 26. A frequency counter 29 is connected to the sweepfrequencysignal generator.

The crystal plating apparatus also includes plating filaments 31A and31B connected through the base of the apparatus to a power supply 32.The bell jar has a vacuum connection 33 to a vacuum pump 34.

In operation, the sweep-frequency signal generator 21 is adjusted tosweep through a band of frequencies of interest that will, normally,include the frequency of the harmonic at which the crystal is intendedto be used. The actual frequency of the generator can be indicated bythe frequency counter 29. The output voltage level of thesweep-frequency generator signal will be constant. The attenuator 23 maybe necessary to reduce the strength of the signal applied to the hybridbridge and to the crystal to a constant level within the tolerance ofthe crystal.

The hybrid bridge applies the constant-level, attenuated,sweep-frequency signal voltage to the crystal, but the impedance of thecrystal, connected to one of the arms of the hybrid bridge, results in avoltage across the crystal, at any specific frequency, that varies withthe effective impedance of the crystal at that frequency. The voltageacross the crystal will rise to a peak at the frequency of each of theharmonic or inharmonic modes of oscillation of the crystal. The relativeheights of the peaks of the fundamental mode or any of its harmonicmodes as compared with those of the inharmonic modes will indicate thequality of the crystal.

The voltage across the crystal is applied to the logarithmic amplifier25 that effectively extends the scale of the signals to be applied tothe oscilloscope 27 and provides greater sensitivity to the lower,inharmonic peaks that are to be reduced by the plating process.

The sweep of the oscilloscope is established and synchronized with thatof the signal generator by the electronic sweep generator 26. This sweepis combined with the signals from the amplifier 25 to provide a trace onthe oscilloscope screen representing the impedance of the crystal withrespect to frequency over the range of the signal generator sweep.

FIG. 2 shows an oscilloscope display tube 28, corresponding to that ofFIG. 1, with a typical example of the display of the impedancevs.frequency characteristics of a crystal over a band of frequenciesincluding one of its harmonics 42. The oscilloscope display tube may beprovided with a grid having an ordinate 40 and an abscissa 41. Thestrength of the signal across the crystal is indicated with respect tothe ordinate 40 and the frequency is shown with respect to the abscissa41. The impedance of the crystal at any frequency is, of course,inversely proportional to this signal strength. The harmonic frequencyof the crystal will have the lowest impedance which, will produce thehighest peak voltage 42. The inharmonic modes of oscillation also affectthe impedance of the crystal as shown by the lower voltage peaks 43.These are normally less than those of the fundamental or harmonicfrequency 42, but still may be high enough to cause erroneousfunctioning of the crystal under critical conditions.

The crystal, mounted in a vacuum in the bell jar, can be plated in awell known manner. The filaments 31A and 31B are charged or loaded withany desired plating material, such as aluminum, gold, copper or silver.Energy is applied to the filaments through suitable connection to thepower supply 32. The plating continues on both sides of the crystal at apredictable rate until the filament is disconnected from the powersupply.

FIG. 3 shows the oscilloscope display tube 28 with another, typicalexample of the impedance vs. frequency characteristics of the samecrystal after an interval of plating. The oscilloscope display tube 28has the same ordinate 40 and abscissa 41 covering the same ranges ofvoltages and frequencies as shown in FIG. 2. The desired harmonicfrequency is now 52. It will be shifted, almost imperceptibly, but itwill not be substantially attenuated.

The plating can be continued until the inharmonic mode peaks 53 havebeen reduced to an acceptable level, or until there are no dominantpeaks that could cause erroneous function of the crystal. The filamentis then disconnected to shut off the plating process and to retain theimproved characteristics of the crystal. The crystal is ready for usewithout further testing.

It should be noted that variations in the sweep frequency test circuitare possible. For example, the attenuator may be omitted if it is notneeded. Other types of test circuit and other types of impedance andfrequency indicators may also be substituted.

In another variation of this crystal plating monitoring system, a peaksensing device may be connected to the output of the hybrid bridge orlogarithmic amplifier, and gated to detect peaks within certain bands offrequencies including those of the most serious of the inharmonic modesof oscillation. This peak sensing device may be set to disconnect thefilament and terminate the plating process when the peaks of theunwanted, inharmonic modes fall below a given level. Alternatively, thedesired harmonic frequency may also be monitored by a peak sensingdevice, and its peak may be compared with the output of the peak sensingdevice for the unwanted, inharmonic modes of oscillation to detect whenthe ratio between the two reaches a given, acceptable proportion beforedisconnecting the filament.

In practice, there may be only a few frequencies that include inharmonicmode peaks of any serious magnitude. These may be monitoredindividually, with or without the oscilloscope display, to ascertainwhen the levels of these particular, unwanted peaks are reduced to anacceptable level.

Perhaps the most ideal system that could be used here to obtain theoptimum quality for each individual crystal, would be to record theprogress of the characteristics of the crystal, with respect to plating,at least for the most dominant inharmonic modes, during the entireplating process, and to plate the crystal to a thickness beyond theapparent optimum. This would provide a record of the changes in thecharacteristics of the crystal with respect to the thickness and a meansfor determining the precise thickness of plating that would provide theoptimum characteristics for any given crystal for any given purpose.This would also establish the minimum plating thickness that wouldprovide acceptable characteristics. The plating could then be etched offand replated to the optimum or desired level.

Each crystal of an identically ground batch may retain a certain degreeof individual characteristics, or respond to plating with certainindividual differences. However, for practical purposes, all of thecrystals in a given batch will follow the same general pattern duringplating. Once an optimum level of plating is established for typicalcrystals of an identical batch the same level can probably be used forthe others as well.

In a typical embodiment of this invention, a crystal ground to a 30Ml-lzfundamental frequency with an intended use at its 150MHz (5th) harmonicwould have a family of inharmonics, extending about 300kHz from the 5thharmonic, and having an apparent strength, only -15db below that of the5th harmonic. When plated with an aluminum electrode of 1.5mm diameterto a thickness whereat the frequency of the 5th harmonic is reducedSOkHz, the apparent strength of the inharmonics are reduced to a levelat least 35db below that of the 5th harmonic.

I wish it to be understood that I do not desire to be limited to theexact details of procedure shown and described, for obviousmodifications will occur to a person skilled in the art.

What is claimed is:

l. A device for plating the surfaces of a crystal while monitoring theelectrical characteristics of said crystal to improve the ratio betweenthe impedance of said crystal at a given harmonic mode of oscillationand the impedances of said crystal at the adjacentinharmonic modes ofoscillation within a given frequency range comprising;

a plating apparatus and an electrical apparatus;

said plating apparatus comprising a vacuum-tight enclosure;

means for evacuating said enclosure;

means for'jmounting said crystal in said enclosure;

a first and second filaments positioned on opposite sides of saidcrystal, said filaments being charged with a plating material;

said electrical apparatus comprising a constantvoltage source of signalsthat covers said given frequency range;

a vacuum-tight electrical connection through said enclosure;

means for connecting said source of signals through said vacuum-tightelectrical connection to said crystal;

means for measuring the voltage of the signal across said crystal oversaid given frequency range;

a source of electrical power;

means for connecting said source of electrical power to said first andsecond filaments to start said plating;

means for disconnecting said source of electrical power from said firstand second filaments when said characteristics of said crystal achieve agiven desired condition; means for determining the ratio between thevoltage of said signal across said crystal at the frequency of saidgiven harmonic mode, and the voltage of said signal across said crystalat the frequencies of said adjacent, inharmonic modes, and said meansfor disconnecting said source of electrical power from said first andsecond filaments operative when said ratio between the voltage of saidsignal across said crystal at the frequency of said given harmonic modeand the voltages of said signals across said crystal at the frequenciesof said adjacent inharmonic modes reaches a given value;

said source of signals comprising a sweep-frequency signal generatorcovering said given frequency range;

said means for connecting said source of signals through saidvacuum-tight electrical connection to said crystal including anattenuator and a hybrid bridge;

said attenuator being connected between said sweepfrequency signalgenerator and said hybrid bridge, and said crystal being connected inone arm of said hybrid bridge; and

said means for measuring the voltage of the signal across said crystalincluding an oscilloscope having a calibrated display tube, means forconnecting the output of said crystal to one of the inputs of saidoscilloscope and means for applying a sweep voltage, synchronized withsaid sweep frequency signal generator, to the other input of saidoscilloscope to indicate the voltage across said crystal at anyfrequency.

2. A device for plating the surfaces of a crystal while monitoring theelectrical characteristics of said crystal as in claim 1 wherein saidvacuum-tight enclosure is a bell jar and said means for evacuating saidenclosure includes a vacuum pump.

3. A device for plating the surfaces of a crystal while monitoring theelectrical characteristics of said crystal as in claim 1 having afrequency counter connected to the output of said sweep frequency signalgenerator to determine the frequency of the output of said sweepfrequency signal generator.

1. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal to improve the ratio between the impedance of said crystal at a given harmonic mode of oscillation and the impedances of said crystal at the adjacent inharmonic modes of oscillation within a given frequency range comprising; a plating apparatus and an electrical apparatus; said plating apparatus comprising a vacuum-tight enclosure; means for evacuating said enclosure; means for mounting said crystal in said enclosure; a first and second filaments positioned on opposite sides of said crystal, said filaments being charged with a plating material; said electrical apparatus comprising a constant-voltage source of signals that covers said given frequency range; a vacuum-tight electrical connection through said enclosure; means for connecting said source of signals through said vacuumtight electrical connection to said crystal; means for measuring the voltage of the signal across said crystal over said given frequency range; a source of electrical power; means for connecting said source of electrical power to said first and second filaments to start said plating; means for disconnecting said source of electrical power from said first and second filaments when said characteristics of said crystal achieve a given desired condition; means for determining the ratio between the voltage of said signal across said crystal at the frequency of said given harmonic mode, and the voltage of said signal across said crystal at the frequencies of said adjacent, inharmonic modes, and said means for disconnecting said source of electrical power from said first and second filaments operative when said ratio between the voltage of said signal across said crystal at the frequency of said given harmonic mode and the voltages of said signals across said crystal at the frequencies of said adjacent inharmonic modes reaches a given value; said source of signals comprising a sweep-frequency signal generator covering said given frequency range; said means for connecting said source of signals through said vacuum-tight electrical connection to said crystal including an attenuator and a hybrid bridge; said attenuator being connected between said sweep-frequency signal generator and said hybrid bridge, and said crystal being connected in one arm of said hybrid bridge; and said means for measuring the voltage of the signal across said crystal including an oscilloscope having a calibrated display tube, means for connecting the output of said crystal to one of the inputs of said oscilloscope and means for applying a sweep voltage, synchronized with said sweep frequency signal generator, to the other input of said oscilloscope to indicate the voltage across said crystal at any frequency.
 2. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal as in claim 1 wherein said vacuum-tight enclosure is a bell jar and said means for evacuating said enclosure includes a vacuum pump.
 3. A device for plating the surfaces of a crystal while monitoring the electrical characteristics of said crystal as in claim 1 having a frequency counter connected to the output of said sweep frequency signal generator to determine the frequency of the output of said sweep frequency signal generator. 